An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD), complimentary metal oxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor, and a passive-pixel sensor.
Another type of pixel is a metal-semiconductor-metal (MSM) photodetector. This type is constructed with two Schottky contacts (e.g., metallic electrodes) on a semiconductor material, instead of a p-n junction as in a photodiode. A voltage potential is applied between the electrodes which creates an electric field in the semiconductor material. The light that is absorbed in the semiconductor material generates electric carriers that are collected by the electric field. These electric carriers contribute to a photocurrent that can be measured and recorded. MSM photodetectors have generally been used in optical telecommunication systems due to their high speed and low parasitic capacitance. However, MSM photodetectors have low responsivity caused by reflection from the surface metal contacts and semiconductor surface.
In order to capture color, image sensors employ a color filter layer that can support several different color filters (e.g., red, green, and blue), and are positioned such that the incident light is directed through the filter. This can be done by using a color filter array mosaic on a single sensor or splitting the incident light with a prism to multiple sensors. Either way, the cost of the color filters used in image sensors accounts for a large part of the total cost of the image sensor. Additionally, the color filter layer adds to the physical size of the device. Improvements in color filter design and fabrication are desired to reduce the cost and size of image sensors.